Strained silicon-on-insulator structures for semiconductor devices combine the benefits of two advanced approaches to performance enhancement: silicon-on-insulator (SOI) technology and strained silicon (Si) technology. The strained silicon-on-insulator configuration offers various advantages associated with the insulating substrate, such as reduced parasitic capacitances and improved isolation. Strained Si provides improved carrier mobilities. Devices such as strained Si metal-oxide-semiconductor field-effect transistors (MOSFETs) combine enhanced carrier mobilities with the advantages of insulating substrates.
Strained-silicon-on-insulator substrates are typically fabricated as follows. First, a relaxed silicon-germanium (SiGe) layer is formed on an insulator by one of several techniques such as separation by implantation of oxygen (SIMOX), wafer bonding and etch back; wafer bonding and hydrogen exfoliation layer transfer; or recrystallization of amorphous material. Then, a strained Si layer is epitaxially grown to form a strained-silicon-on-insulator structure, with strained Si disposed over SiGe. The relaxed-SiGe-on-insulator layer serves as the template for inducing strain in the Si layer. This induced strain (i.e., a dimensionless value indicating the change in gauge length of a sample, in the direction of an applied stress, divided by its original gauge length) is typically greater than 10−3.
This structure has limitations. It is not conducive to the production of fully-depleted strained-silicon-on-insulator devices in which the layer over the insulating material must be thin enough (<300 angstroms [Å]) to allow for full depletion of the layer during device operation. Fully depleted transistors may be the favored version of SOI for MOSFET technologies beyond the 90 nm technology node. The relaxed SiGe layer adds to the total thickness of this layer and thus makes it difficult to achieve the small thicknesses required for fully depleted SOI device fabrication. The relaxed SiGe layer is not required, however, if a strained Si layer can be produced directly on the insulating material. Thus, there is a need for a method to produce strained silicon—or other semiconductor—layers directly on insulating substrates.
At the same time, a uniformly strained layer may not be preferable for the formation of different types of devices on a single substrate. In addition, a pre-amorphization implant (PAI) that may be performed to improve device characteristics may result in strained layer relaxation, leading to a loss of carrier mobility enhancement.